diff --git a/documentation/wiki/ChangeWaves.md b/documentation/wiki/ChangeWaves.md
index c09ef8f9bf9..66693a74667 100644
--- a/documentation/wiki/ChangeWaves.md
+++ b/documentation/wiki/ChangeWaves.md
@@ -24,6 +24,9 @@ A wave of features is set to "rotate out" (i.e. become standard functionality) t
 
 ## Current Rotation of Change Waves
 
+### 18.4
+- [Start throwing on null or empty paths in MultiProcess and MultiThreaded Task Environment Drivers.](https://github.com/dotnet/msbuild/pull/12914)
+
 ### 18.3
 - [Replace Transactional property with ChangeWave control, implement atomic file replacement with retry logic, and update tests.](https://github.com/dotnet/msbuild/pull/12627)
 
diff --git a/src/Build/BackEnd/TaskExecutionHost/MultiProcessTaskEnvironmentDriver.cs b/src/Build/BackEnd/TaskExecutionHost/MultiProcessTaskEnvironmentDriver.cs
index f794bf64293..bec24b034f7 100644
--- a/src/Build/BackEnd/TaskExecutionHost/MultiProcessTaskEnvironmentDriver.cs
+++ b/src/Build/BackEnd/TaskExecutionHost/MultiProcessTaskEnvironmentDriver.cs
@@ -4,11 +4,6 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
-#if NETFRAMEWORK
-using Microsoft.IO;
-#else
-using System.IO;
-#endif
 using Microsoft.Build.Framework;
 using Microsoft.Build.Internal;
 
@@ -45,10 +40,13 @@ public AbsolutePath ProjectDirectory
         /// <inheritdoc/>
         public AbsolutePath GetAbsolutePath(string path)
         {
-            // This function should not throw when path has illegal characters.
-            // For .NET Framework, Microsoft.IO.Path.Combine should be used instead of System.IO.Path.Combine to achieve it.
-            // For .NET Core, System.IO.Path.Combine already does not throw in this case.
-            return new AbsolutePath(Path.Combine(NativeMethodsShared.GetCurrentDirectory(), path), ignoreRootedCheck: true);
+            // Opt-out for null path when Wave18_4 is disabled - return null as-is.
+            if (!ChangeWaves.AreFeaturesEnabled(ChangeWaves.Wave18_4) && path is null)
+            {
+                return new AbsolutePath(path!, path!, ignoreRootedCheck: true);
+            }
+
+            return new AbsolutePath(path, basePath: ProjectDirectory);
         }
 
         /// <inheritdoc/>
diff --git a/src/Build/BackEnd/TaskExecutionHost/MultiThreadedTaskEnvironmentDriver.cs b/src/Build/BackEnd/TaskExecutionHost/MultiThreadedTaskEnvironmentDriver.cs
index cf54c430d0d..1bf9009054a 100644
--- a/src/Build/BackEnd/TaskExecutionHost/MultiThreadedTaskEnvironmentDriver.cs
+++ b/src/Build/BackEnd/TaskExecutionHost/MultiThreadedTaskEnvironmentDriver.cs
@@ -71,6 +71,12 @@ public AbsolutePath ProjectDirectory
         /// <inheritdoc/>
         public AbsolutePath GetAbsolutePath(string path)
         {
+            // Opt-out for null path when Wave18_4 is disabled - return null as-is.
+            if (!ChangeWaves.AreFeaturesEnabled(ChangeWaves.Wave18_4) && path is null)
+            {
+                return new AbsolutePath(path!, path!, ignoreRootedCheck: true);
+            }
+
             return new AbsolutePath(path, ProjectDirectory);
         }
 
diff --git a/src/Framework/ChangeWaves.cs b/src/Framework/ChangeWaves.cs
index 66ff885a352..22049fdfc65 100644
--- a/src/Framework/ChangeWaves.cs
+++ b/src/Framework/ChangeWaves.cs
@@ -31,7 +31,8 @@ internal static class ChangeWaves
         internal static readonly Version Wave17_12 = new Version(17, 12);
         internal static readonly Version Wave17_14 = new Version(17, 14);
         internal static readonly Version Wave18_3 = new Version(18, 3);
-        internal static readonly Version[] AllWaves = [Wave17_10, Wave17_12, Wave17_14, Wave18_3];
+        internal static readonly Version Wave18_4 = new Version(18, 4);
+        internal static readonly Version[] AllWaves = [Wave17_10, Wave17_12, Wave17_14, Wave18_3, Wave18_4];
 
         /// <summary>
         /// Special value indicating that all features behind all Change Waves should be enabled.
diff --git a/src/Framework/PathHelpers/AbsolutePath.cs b/src/Framework/PathHelpers/AbsolutePath.cs
index 1c5cc585e5b..7fb31c57232 100644
--- a/src/Framework/PathHelpers/AbsolutePath.cs
+++ b/src/Framework/PathHelpers/AbsolutePath.cs
@@ -19,7 +19,7 @@ namespace Microsoft.Build.Framework
     /// file system conventions (case-sensitive on Linux, case-insensitive on Windows and macOS).
     /// Does not perform any normalization beyond validating the path is fully qualified.
     /// A default instance (created via <c>default(AbsolutePath)</c>) has a null Value 
-    /// and should not be used. Two default instances are considered equal.
+    /// and represents an issue in path handling. Two default instances are considered equal.
     /// </remarks>
     public readonly struct AbsolutePath : IEquatable<AbsolutePath>
     {
@@ -32,6 +32,11 @@ namespace Microsoft.Build.Framework
         /// The normalized string representation of this path.
         /// </summary>
         public string Value { get; }
+        
+        /// <summary>
+        /// The original string used to create this path.
+        /// </summary>
+        public string OriginalValue { get; }
 
         /// <summary>
         /// Initializes a new instance of the <see cref="AbsolutePath"/> struct.
@@ -41,6 +46,7 @@ public AbsolutePath(string path)
         {
             ValidatePath(path);
             Value = path;
+            OriginalValue = path;
         }
 
         /// <summary>
@@ -50,12 +56,24 @@ public AbsolutePath(string path)
         /// <param name="ignoreRootedCheck">If true, skips checking whether the path is rooted.</param>
         /// <remarks>For internal and testing use, when we want to force bypassing the rooted check.</remarks>
         internal AbsolutePath(string path, bool ignoreRootedCheck)
+            : this(path, path, ignoreRootedCheck)
         {
-            if (!ignoreRootedCheck) 
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="AbsolutePath"/> struct.
+        /// </summary>
+        /// <param name="path">The absolute path string.</param>
+        /// <param name="original">The original string used to create this path.</param>
+        /// <param name="ignoreRootedCheck">If true, skips checking whether the path is rooted.</param>
+        internal AbsolutePath(string path, string original, bool ignoreRootedCheck)
+        {
+            if (!ignoreRootedCheck)
             {
                 ValidatePath(path);
             }
             Value = path;
+            OriginalValue = original;
         }
 
         /// <summary>
@@ -85,7 +103,14 @@ private static void ValidatePath(string path)
         /// </summary>
         /// <param name="path">The path to combine with the base path.</param>
         /// <param name="basePath">The base path to combine with.</param>
-        public AbsolutePath(string path, AbsolutePath basePath) => Value = Path.Combine(basePath.Value, path);
+        public AbsolutePath(string path, AbsolutePath basePath)
+        {
+            // This function should not throw when path has illegal characters.
+            // For .NET Framework, Microsoft.IO.Path.Combine should be used instead of System.IO.Path.Combine to achieve it.
+            // For .NET Core, System.IO.Path.Combine already does not throw in this case.
+            Value = Path.Combine(basePath.Value, path);
+            OriginalValue = path;
+        } 
 
         /// <summary>
         /// Implicitly converts an AbsolutePath to a string.